Counterweighted roof for in-ground reservoirs for liquefied gases



@ct. 22, 1968 M. NACHSHEN 3,406,859

COUNTERWEIGHTED ROOF FOR INGROUND RESERVOIRS FOR LIQUEFIED GASES Filed Jan. 10, 1967 a. I; IN In? j N INVENT OR Maurice Nochshen ATTORNEY United States Patent 3,406,859 COUNTERWEIGHTED ROOF FOR IN-GROUND RESERVOIRS FOR LIQUEFIED GASES Maurice Nachshen, London, England, assignor t0 Conch International Methane Limited, Nassau, Bahamas, a Bahamian company Filed Jan. 10, 1967, Ser. No. 608,405 Claims priority, application Great Britain, Feb. 24, 1966, 8,156/ 66 Claims. (Cl. 220-18) ABSTRACT OF THE DISCLOSURE The roofs of ground reservoirs for storing liquefied gases have to be held down in some way because of the lifting pressure on the roof arising from boil-off of the stored gas. The present disclosure describes hold-down means for such a roof wherein the roof comprises a thin membrane sheet supported from a superstructure, with counterweights, also supported by the superstructure, above the membrane in such fashion as to exert substantially no downward thrust on the membrane, but to support the membrane against upward thrust due to internal gas pressure.

This invention relates to a roof suitable for an inground reservoir for storing cold liquids.

Ground reservoirs are particularly suitable for storing a liquefied gas which boils at atmospheric pressure at a temperature below the ambient temperature. Liquefied nitrogen, liquefied air, liquefied methane, liquefied natural gas, liquefied oxygen and liquefied propane can be cited as examples of such products.

Hitherto when storing many liquefied gases in inground reservoirs the roofs have been anchored around the periphery of the reservoir because of the lifting pressures on the roof arising from the boil-off of the gas. Such anchorage has usually entailed a large mass of concrete at the periphery and long anchor bolts. Moreover, in the case of large reservoirs it has usually been deemed expedient to make the roof and superstructure of costly metal which does not become brittle at the low temperatures of the liquefied gas.

It has already been proposed, as shown in US. Patent No. 3,249,251, to overcome these disadvantages by means of a roof which comprises a flat or cambered sheet lined on at least one surface with thermal insulation, a superstructure to which the sheet is attached and a counterweight acting on this sheet. However, the sheet which is attached to the superstructure is preferably of thin metal and desirably a metal is employed which does not become embrittled at very low temperatures, for instance a high nickel steel, a stainless steel, copper, aluminum or aluminum alloys. Such a thin sheet when covering a considerable area occupied by an in-ground reservoir has a very limited capacity to withstand stresses and loads. Accordingly the provision of a counterweight adequate to deal with upward pressure surges on the sheet involves a much more robust construction for the roof and this leads to greatly increased cost.

The present invention stems in part from a recognition of the circumstance that the provision of a counterweight is only called for on those occasions when the reservoir contains a boiling liquid and the sheet constituting the roof of the reservoir is therefore subject to upward pressures from boil-off of liquefied gases in the reservoir. Moreover, the downward resistance of the counterweight is then needed only in proportion to the lifting pressures on the thin membrane; the desideraturn of economy in construction in the concept of an in-ground reservoir for storing liquefied gases makes it necessary to look for a 3,406,859 Patented Oct. 22, 1968 construction which is also relatively inexpensive in terms of materials and of constructional labor.

According to the present invention, in a roof suitable for an in-ground reservoir comprising a flat orcambered sheet lined on at least one surface with thermal insulation and attached to the superstructure, a plurality of counterweights are suspended from the superstructure by a system of supports such as substantially to relieve the sheet from the weight of the counterweights which act as a substantially continuous reaction to the upward pressure of the gas on the underside of the sheet.

Such a roof preferably has a system of supports for the counterweights so arranged that each support carries at least part of the load of adjacent counterweights. For example, the supports may take the form of a casting or a rigid assembly of a horizontal plate and vertical ribs or inclined bracing stays arranged to support the adjacent corners of several counterweights. The latter may conform to some regular pattern which lends itself to support at the corners; for instance the counterweights may be square, rectangular or hexagonal. The aforesaid supports may hang directly from the superstructure or they may be attached to the superstructure by hangers which permit some horizontal movement.

As already indicated in Patent No. 3,249,251, the roof constructed from thin sheet metal may be supported by a system of metal stiffening ribs extending in one or more directions and such ribs may have for instance sufficient additional strength to carry a workman and light plant for any occasional maintenance. Instead of providing a separate means of attachment to the superstructure for the thin metal sheet roof and its supporting ribs, the latter may be attached to the underside of the counterweight supports. Alternatively the thin metal sheet roof may be attached to the underside of the counterweight supports.

Advantageously there is interposed between the lower side of the counterweights and the thin metal sheet roof a layer or layers of load-bearing filling material which may in itself also have some thermal insulation value. Such layer or layers can serve to absorb and transmit upward thrust on the thin metal sheet and may perhaps also counter any tendency of the thin metal sheet to buckle. As suitable layers for this purpose there may be used plastic material such as foamed polystyrene, foamed polyurethane, foamed polyvinylchloride, or alternatively cork may be used. If desired there may additionally be interposed between the bottom of the counterweights and the thin metal sheet a layer of thermal insulation. Such a provision may obviate the need for any further heat insulation barriers to protect the superstructure supporting the counterweights.

In any event the thin metal sheet roof is provided with thermal insulation on at least one side and it is desirable this should be the underside exposed to the cold. Such thermal insulation may be of fibre glass, foamed plastic, balsa wood, paper honeycomb, kapok, mineral wool or rock wool. Generally some support for the thermal insulation lining on the underside of the sheet will be required. A very suitable support is a wire mesh grid but alternative supports include plywood sheets, wallboard, fibre board, wood, wool or similar materials, preferably suspended from the roof sheet at or adjacent to the points where the latter is in turn supported for instance by the counterweight supports.

The superstructure may comprise trusses, girders, or beams, either straight or curved, which may be spaced at intervals across the roof and may intersect each other to form a grid. The superstructure member may all meet one another at the center in the case of a circular roof, thereby acting as radial members, as shown in Patent No. 3,249,251. The superstructure may alternatively be made up from girders, beams or trusses of any suitable material, for instance metal or wood. The material need not be one which is free from cold embrittlement. It will, of course, be evident that part of the superstructure must extend beyond the sheet so that the system of counterweights and the thin sheet roof which must cover the opening of the reservoir, can be effectively supported.

The counterweights may be concrete slabs cast in any convenient form; for example they may be square, rectangular or hexagonal slabs and if desired they may be supported on a rigid base or in a frame built up from metal angles. Such a support or frame may be of advantage in avoiding trouble otherwise arising from a fracture or spalling of a portion of the slab. Alternatively the counterweights may take the form of rigid trays loaded with sand or gravel. Preferably the counterweights should be arranged so as to provide a regularly distributed downward thrust to the sheet when it is subjected to lifting pressures. The magnitude of the counterweight load should not greatly exceed that which is necessary to deal with the maximum pressure arising from boil-01f of the liquefied gases in order to limit the load placed on the superstructure when there is no gas pressure.

The roof may be fitted with liquid inlet, liquid and vapor outlet and safety outlet valves and additionally may be provided with a gas seal. A suitable form of gas seal has already been described in US. Patent No. 3,175,370.

The specific nature of the invention, as well as other objects and advantages thereof, will clearly appear from a description of a preferred embodiment as shown in the accompanying drawing in which:

FIG. 1 shows a plan view of a superstructure for a circular in-ground reservoir for liquefied gases;

FIG. 2 shows a cross sectional elevation of the roof and supported counterweight system suspended from a superstructure;

FIG. 3 shows a part of the elevation in FIG. 2 on a larger scale and in greater detail;

FIG. 4 shows in perspective one of the counterweight supports involved in the construction illustrated in FIGS. 1 and 2; and

FIG. 5 shows the details of a hanger connection.

The superstructure for an in-ground reservoir for storing liquefied gas comprises a rectangular grid of steel members wherein the members 2 are attached to the underside of the members 1, the latter being supported on columns 3, mounted on a concrete bed 4, or alternatively on a piled foundation, the whole resting on the earth surrounding and defining the in-ground reservoir 4a, of known construction.

Attached to the members 2 of the superstructure are steel hangers 5 which at their lower ends are attached to rings 7 of counterweight supports 6. The effective length of each hanger 5 may be adjusted by turning a nut 5a at its upper end. The nut should be provided with a washer 5b, the contact surface between nut and washer being spherical to permit small changes of angle of the hanger relative to the superstructure. A wooden block 50 may be provided as a thermal break. The counterweight supports 6 comprise a steel base plate 8 reinforced by vertical ribs 9 which divide the base plate into four segments. The hangers 5 and supports 6 are located and arranged to support the adjacent corners of four counterweights, these consisting of identical rectangular concrete slabs 10 which may be enclosed, if desired, in a frame fabricated from metal angle. Each counterweight 10 may be provided with a lifting loop 10a and the counterweights are arranged in a pattern whereby each segment of the base plate 8 supports a corner of a diflFerent counterweight. In this way the arrangement provides that the system of counterweights 10 is carried by a series of counterweight supports 6, each depending from hangers 5 attached to the superstructure.

Immediately below the base plate 8, the roof membrane 12 is located, and this may be formed from thin aluminum or aluminum alloy sheet or other material which is not subject to cold embrittlement. This membrane 12 may be attached to the underside of the base plate 8 or be otherwise connected to the supports 6, for instance by hanger rods passing through a hollow center in the supports 6. The membrane 12 is provided with a series of parallel or intersecting stiffening ribs 13 and a gas seal is formed by the edge of the membrane being turned down into the annular channel 14 carrying a liquid which is frozen under the temperatures prevailing with storage of cold liquefied gases. Immediately below the counterweight 10 and above the membrane 12 a filling layer of load-bearing material 11 is inserted and this may advantageously consist of thermal insulation material, as for instance foamed polyurethane. This filling material will accommodate itself to small variations in the depth of space between the membrane and the counterweights, the primary object being to ensure that the gas pressure is applied substantially evenly through the membrane and the filling to the counterweights.

The underside of the membrane 12 is thermally insulated by a layer of thermal insulation material 15 such as balsa wood, fiber glass or the like, and this is maintained in position by a wire mesh grid 16 which is held by tie rods 17 to the stiffening ribs 13. All the members below the membrane should be fabricated from material which is not subject to cold embrittlement.

The arrangement provides that the roof membrane 12 is under no load from the counterweights. Upward pressure on the membrane, however, is transmitted to the counterweight load.

In the arrangement described, expensive materials not subject to cold embrittlement, such as aluminum alloys, are needed only in fabricating the membrane, its immediate stiffening system and the light supporting members to the insulation under the membrane. None of these are required to resist substantial gas pressures. Such gas pressures are resisted by the counterweights of cheap material. When there is no gas pressure the load constituted by the counterweights and by all the other members described above is carried by a supporting structure of conventional materials of modest cost, for example mild steel and concrete.

However, the system as described may be so constructed that any substantial conduction of heat from the superstructure to the membrane is avoided and thus the need for a cold break can be eliminated. However if desired such a cold break can be inserted by interrupting the direct connection between the counterweight supports and the superstructure; thus the hangers suspended from the superstructure may be provided with a block 50 of thermal insulation.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of my invention as defined in the appended claims.

I claim:

1. A roof for an in-ground reservoir for a boiling liquid at low temperature comprising (a) a load-bearing superstructure supported by the ground,

(b) a gas-impervious thin lightweight membrane sheet covering the reservoir in gas-tight fashion,

(c) a plurality of individual supporting means supporting said membrane from said superstructure,

(d) a plurality of counterweight units also supported by said supporting means directly above and adjacent to said membrane, and supporting the membrane against any upward thrust on the lower surface of the membrane, substantially the entire weight of the counterweight units being taken by said supporting means so that they exert no substantial downward force on the membrane in the 'unpressurized condition of the reservoir.

2. A roof as claimed in claim 1, including a layer of load-bearing material between said counterweight units and said membrane for uniformly transmitting upward gas pressure from said membrane to said counterweight units.

3. The invention according to claim 2, said layer being a layer of insulating material.

4. The invention according to claim 3, and a main layer of insulating material supported from said supporting means below said membrane.

5. The invention according to claim 1, said counterweight units extending substantially across the entire area of the membrane.

6. The invention according to claim 1, said individual supporting means comprising individual hanger rods suspended from said superstructure.

7. The invention according to claim 6, and means for individually adjusting the length of the hanger rods.

8. The invention according to claim 6, and rigid metal holders attached to said hanger rods, each of said holders having vertical ribs attached to a bottom plate.

9. The invention according to claim 8, said counterweight units being polygonal concrete slabs, the corners of which rest on said bottom plate between two adjacent ones of said vertical ribs.

10. The invention according to claim 9, said superstructure being an open metal framework.

References Cited UNITED STATES PATENTS 3,249,251 5/1966 Nachshen 220l8 3,352,443 11/1967 Sattelberg et al. 220l8 X FOREIGN PATENTS 968,070 8/1964 Great Britain.

THERON E. CONDON, Primary Examiner. G. E. LOWRANCE, Assistant Examiner. 

